May 20, 2021 – Weather system moving into region will bring cooler temperatures, foothill and mountain showers along with a few afternoon thunderstorms through this weekend. Dry with breezy north winds in the valley today and Friday. The higher elevations will see some late season snowfall accumulations. Dry and milder weather returns early next week.
Discussion
Low pressure system is moving into the region and is currently producing some showers over eastern Shasta county and the northern Sierra from around Plumas county northward. As the low deepens today activity will spread further south to around I-80 by late morning and then spread south along the crest down through Tuolumne county in the afternoon.
Snow levels will drop this morning and while some snow falling may be observed down to 3500 to 4000 feet any accumulations of an inch or higher will remain above the 5000 to 6000 foot elevation. Some showers and isolated thunderstorms may also develop along the coastal range today with some models hinting at some showers drifting south through southern Lake county, Napa and western Solano counties late in the afternoon.
The central part of the low will move over Nevada on Friday with wrap around moisture spreading over the Sierra Nevada with more snow showers likely. Dry with breezy north winds will continue for the valley with temperatures becoming warmer but still cooler than seasonal normals. Snow levels will rise during the day to near pass levels before lowering in the evening to around 6000 feet.
It will take until Sunday before the low moves far enough east for the showers to diminish over the mountains with Saturday a repeat of Friday but with higher snow levels ranging from 6500 to 7500 feet. I have left a mention of a few pop up showers Sunday afternoon near the crest otherwise expect dry and warmer conditions.
Extended discussion (Monday through Thursday)
Dry with weak ridging and warmer temperatures to start the work week with temperatures warming into the 85 to 90 range in the valley. The warmup will be short lived for most areas as a low pressure system moves through the region on Tuesday.
Models are mixed with the progression of the low but cluster analysis and the European match up fairly well with the trough staying over the region for Tuesday and Wednesday. Confidence is low with temperatures and precipitation chances and while the NBM has conditions dry at this time if this does develop then changes will likely occur to the forecast. The low should progress eastward enough by Thursday to bring warmer and dry conditions to the interior.